In recent years, size reduction, thickness reduction, and cost reduction in a variety of transformers have been promoted. The same matters are required of a transformer designed for a high voltage as well. Sheet type transformers are advantageous as a low-power thin-type transformer to be used in small apparatuses. A sheet type transformer is made up by, for example, superposing a secondary coil formed by winding an insulated conductive wire in the form of a spiral on a primary coil formed by stamping a sheet of conductive plate into a spiral shape, and fixing the secondary coil on the first coil with an adhesive. Such sheet type transformers are disclosed in the following Patent Documents 1, 2, and 3. Further, miniaturized transformers designed for a high voltage are disclosed in the following Patent Documents 4 and 5, for example.
The sheet type transformer disclosed in Patent Document 1 is made up by forming one winding with a dielectric-coated spiral conductive wire and the other winding by use of a pattern of a printed circuit board, and further fixing both of the windings to each other with a tape. However, in the sheet type transformer disclosed in Patent Document 1, the conductive wire wound in a spiral form is covered with an insulating layer. In a transformer for generating a high voltage, the dielectric-coated layer of a conductor wire for securing a withstand voltage against a high voltage at the output could be thicker, and thus the transformer for generating a high voltage that requires a large number of coil turns in a secondary winding thereof could be larger in size.
In the sheet type transformer disclosed in Patent Document 2, one winding is composed of a three-layer-insulated spiral conductor wire, while the other winding is composed by stamping a conductive plate, and the one winding is superposed on the other. However, in the sheet type transformer disclosed in Patent Document 2, the conductor wire wound in a spiral form is covered with three insulating layers. When the transformer is used as transformers for generating a high voltage, the withstand voltage of the three insulating layers thereof determines the limit of the withstand voltage of the transformer.
In the sheet type transformer disclosed in Patent Document 3, a primary winding and a secondary winding are wounded in a uniplanar spiral shape where the primary winding is internally disposed and the secondary winding is externally disposed, and the lead wires of both the windings are disposed in different positions. However, in the sheet type transformer disclosed in Patent Document 3, the withstand voltage between the primary winding and the secondary winding is secured by the withstand voltage of each conductor wire, and thus the sheet type transformer is inapplicable to transformers generating a high voltage exceeding the withstand voltage of the conductor wire.
The transformer disclosed in Patent Document 4 is a step-up transformer, a uniplanar bobbin has a primary winding wound on the inside thereof and has a secondary winding wound on the outside thereof, and the lead wire of each winding is embedded in a slit which is provided in the bobbin and used for each winding with an insulating adhesive. In the step-up transformer disclosed in Patent Document 4, the insulating adhesive embedding the lead wire therein serves the function of the insulating member securing the withstand voltage, and the withstand voltage of the transformer is determined by the thickness of the adhesive. However, the step of filling the adhesive thereinto involves some uncertain factors in quality such as the remainder of voids and the excessive or deficient injection amounts of the adhesive. Therefore, in order to provide the transformer with a sufficient withstand voltage, the adhesive has to be filled to a substantial thickness. This requires a deeper slit for forming the filling depth, a large thickness of the base of the bobbin (causing a larger size thereof), and a large amount of the adhesive to be filled as a matter of course, thus making it difficult to secure the stable quality. For this reason, the structure of such a step-up transformer is inapplicable to compact high-voltage generating transformers.
The transformer disclosed in Patent Document 5 is a high-voltage transformer, and has a structure where a uniplanar bobbin (base) has a primary winding wound on the outside thereof and has a secondary winding wound on the inside thereof, the lead wire of the secondary winding is routed down to the groove (lead wire drawing-out groove) provided in the bobbin and is drawn out to a terminal, and the partition of an upper guard is to be fit to the partition of the base enclosing the secondary winding. In the transformer, the magnitude of the withstand voltage is determined by the depth of the groove where the lead is routed down and the creeping distance where the partition provided on the base overlaps with the partition provided on the guard. If those depth and distance are increased, the transformer is increased in size as a matter of course. Therefore, the structure of such a transformer is inapplicable to compact high-voltage transformers.
Patent Document 1: JP-A-1996-316040
Patent Document 2: JP-A-1996-306539
Patent Document 3: JP-A-1997-199347
Patent Document 4: JP-A-1994-112065
Patent Document 5: JP-A-1994-342726
In view of the above-cited documents, there should be developed a compact transformer designed for a high-voltage satisfying the following requirements:
the degree of coupling between a primary winding and a secondary winding is enhanced (the energy of the primary winding is efficiently transmitted to the secondary winding);
the cross-sectional areas of wire materials of the primary winding and the secondary winding are sufficiently large (the loss at the time of energizing of the transformer is reduced by reducing the electric resistance thereof); and
the transformer is manufactured at low cost (the materials are inexpensive, the number of parts is small, and the manufacturing process is simple).
The above-described sheet type transformer is effective in performing a compact and thickness-reduced transformer. However, when a transformer designed for a high voltage is built by use of a sheet type transformer, there are the following problems because of a slimness of the sheet type transformer as a feature:
It is difficult to ensure insulating properties and withstand voltages in an area where a high potential difference is generated between starting and ending points of a winding for a high voltage; and
It is difficult to obtain the insulating properties and the withstand voltages between the members such as winding and terminal on the low voltage side, and the area where a high voltage is generated.
The present invention provides a sheet type transformer with a simple structure, causing no damage to its slimness, and securing high insulating properties to address a high voltage.